Most Recent Bulletin Report: June 1992 (BGVN 17:06)

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Lassen Report. Of the three major Holocene volcanoes in the California Cascades, Lassen (~800 km NNW of the epicenter) had the strongest response to the 28 June earthquake (figure 1). About 10 minutes after the S-wave's arrival and while surface waves were still being recorded, a M 2.8 event occurred south of Lassen Peak. Film records showed 9 more earthquakes in the first hour, and 22 events were identified during the first 24 hours. Although most were M 1 or smaller, at least two and perhaps as many as four were of magnitude greater than or equal to 2. Nine were detected by the RTP system. The best preliminary locations were concentrated ~3 km SW of Lassen Peak at <5 km depth beneath Mt. Diller, one of the most active areas around Lassen, where small clusters of shallow M 1-2 events occur every 1-2 months. Three more small events occurred SW of Lassen Peak two days after the M 7.5 earthquake, with two additional events on 5 July.

Figure 1. Seismic events in the Lassen area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of S. Walter.

All information contained in these reports is preliminary and subject to change.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Lassen Report. Of the three major Holocene volcanoes in the California Cascades, Lassen (~800 km NNW of the epicenter) had the strongest response to the 28 June earthquake (figure 1). About 10 minutes after the S-wave's arrival and while surface waves were still being recorded, a M 2.8 event occurred south of Lassen Peak. Film records showed 9 more earthquakes in the first hour, and 22 events were identified during the first 24 hours. Although most were M 1 or smaller, at least two and perhaps as many as four were of magnitude greater than or equal to 2. Nine were detected by the RTP system. The best preliminary locations were concentrated ~3 km SW of Lassen Peak at <5 km depth beneath Mt. Diller, one of the most active areas around Lassen, where small clusters of shallow M 1-2 events occur every 1-2 months. Three more small events occurred SW of Lassen Peak two days after the M 7.5 earthquake, with two additional events on 5 July.

Figure 1. Seismic events in the Lassen area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of S. Walter.

Geological Background

The Lassen volcanic center consists of the andesitic Brokeoff stratovolcano SW of Lassen Peak, a dacitic lava dome field, and peripheral small andesitic shield volcanoes and large lava flows, primarily on the Central Plateau NE of Lassen Peak. A series of eruptions from Lassen Peak from 1914 to 1917 marks the most recent eruptive activity in the southern Cascade Range. Activity spanning about 825,000 years began with eruptions of the Rockland caldera complex and was followed beginning about 590,000 years ago by construction of Brokeoff stratovolcano. Beginning about 310,000 years ago activity shifted to the north flank of Brokeoff, where episodic, more silicic eruptions produced the Lassen dome field, a group of 30 dacitic lava domes including Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading Peak. At least 12 eruptive episodes took place during the past 100,000 years, with Lassen Peak being constructed about 27,000 years ago. The Chaos Crags dome complex was constructed about 1100-1000 years ago north of Lassen Peak. The Cinder Cone complex NE of Lassen Peak was erupted in a single episode several hundred years before present and is considered part of the Lassen volcanic center (Clynne et al., 2000). The 1914-1917 eruptions of Lassen Peak began with phreatic eruptions and included emplacement of a small summit lava dome, subplinian explosions, mudflows, and pyroclastic flows.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Brokeoff Mountain

Stratovolcano

2815 m

40° 27' 0" N

121° 33' 0" W

Cinder Cone
Black Butte

Pyroclastic cone

2105 m

40° 33' 0" N

121° 19' 0" W

Hat Mountain

Cone

2345 m

40° 30' 0" N

121° 25' 0" W

Prospect Peak

Shield volcano

2541 m

40° 34' 0" N

121° 21' 0" W

Raker Peak

Cone

2281 m

40° 32' 0" N

121° 28' 0" W

West Prospect Peak

Shield volcano

2490 m

40° 36' 0" N

121° 23' 0" W

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Crescent Crater

Crater

2635 m

40° 30' 0" N

121° 30' 0" W

Domes

Feature Name

Feature Type

Elevation

Latitude

Longitude

Bumpass Mountain

Dome

2668 m

40° 28' 0" N

121° 30' 0" W

Chaos Crags

Dome

2592 m

40° 31' 0" N

121° 31' 0" W

Eagle Peak

Dome

2811 m

40° 29' 0" N

121° 31' 0" W

Helen, Mount

Dome

Lassen Peak
St. Joseph's, Mount
Saint Joseph's, Mount

Dome

3187 m

40° 29' 31" N

121° 30' 29" W

Reading Peak

Dome

2652 m

40° 28' 0" N

121° 28' 0" W

Ski Heil Peak

Dome

2706 m

40° 29' 0" N

121° 31' 0" W

Sunflower Flat

Dome

2222 m

40° 33' 0" N

121° 31' 0" W

Vulcans Castle

Dome

2570 m

40° 29' 0" N

121° 32' 0" W

Thermal

Feature Name

Feature Type

Elevation

Latitude

Longitude

Boiling Springs Lake

Hot Spring

40° 26' 0" N

121° 24' 0" W

Bumpass Hell

Thermal

2410 m

40° 28' 0" N

121° 30' 0" W

Devils Kitchen

Thermal

40° 26' 30" N

121° 26' 0" W

Domingo Spring

Hot Spring

40° 22' 0" N

121° 21' 0" W

Drakesbad

Thermal

40° 26' 30" N

121° 25' 0" W

Growler Hot Springs

Hot Spring

40° 23' 30" N

121° 30' 0" W

Little Hot Springs Valley

Hot Spring

40° 27' 0" N

121° 31' 0" W

Morgan Hot Springs

Hot Spring

40° 23' 0" N

121° 31' 0" W

Sulfur Works

Thermal

2170 m

40° 27' 0" N

121° 32' 0" W

Terminal Geyser

Geyser

40° 25' 0" N

121° 23' 0" W

Photo Gallery

The blocky surface of Chaos Jumbles in the foreground was produced by collapse of a portion of the Chaos Crags lava dome complex in the background about 1650 AD. The cold rock avalanche traveled 5 km from its source, and may have occurred in the absence of any eruptive activity. Three successive lobes of the avalanche cover an area of 8 sq km.

Photo by Lee Siebert, 1982 (Smithsonian Institution).

The tree stump and strip of red bark fragments in the foreground are the remnants of one of the many trees blown down radially away from the volcano by a pyroclastic surge on May 22, 1915, during the paroxysmal phase of the 1914-1917 eruption of Lassen Peak. The Devastated Area in the background remained sparsely vegetated for many decades following the eruption.

Photo by Lee Siebert, 1982 (Smithsonian Institution).

The trunks of the large ponderosa pine tree at the left and the smaller birch to the right in the Lost Creek valley NE of Lassen Peak were bent over by a mudflow from an eruption in May 1915. After the eruption vertical growth of the trees resumed. The mudflows traveled 50 km from the volcano, destroying bridges, farmlands, and farm buildings.

Photo by Lee Siebert, 1982 (Smithsonian Institution).

Chaos Jumbles, a debris avalanche-deposit formed by collapse of Chaos Crags about 1650 AD, is seen from the top of the avalanche scarp. The avalanche traveled up to 5 km in three lobes now covered with varying degrees of vegetation. The avalanche was emplaced as a cold rockfall avalanche; there is no direct evidence for an associated explosive eruption.

Photo by Lee Siebert, 1982 (Smithsonian Institution).

The massive dacitic lava dome of Lassen Peak rises above Lake Helen on the south side of the volcano. Plugs of dark dacitic lava exposed near the summit are surrounded by vast aprons of lighter-colored talus associated with growth of the lava dome.

Photo by Lee Siebert, 1972 (Smithsonian Institution).

Hikers perch on a small pinnacle of dacitic lava that forms the high point of Lassen Peak, the southernmost major volcano in the Cascade Range that stretches from southern British Columbia to northern California. Lassen Peak is a large dacitic lava dome that last erupted from 1914 to 1917.

Photo by Lee Siebert, 1968 (Smithsonian Institution).

An aerial view of cloud-capped Lassen Peak from the NW shows the Chaos Crags lava dome complex on its left, the source of rockfall avalanche in 1650 AD, and the older Brokeoff stratovolcano and post-caldera lava domes on the right. Lassen Peak was the source of California's latest eruption, that lasted from 1914 to 1917.

Photo by Lyn Topinka, 1984 (U.S. Geological Survey).

The sparsely vegetated Devastated Zone on the NE flank of Lassen Peak was swept by a pyroclastic surge on May 22, 1915, during the paroxysmal phase of the 1914-1917 eruption. The pyroclastic surge destroyed forests, and was accompanied by mudflows that traveled down Lost Creek and Hat Creek valleys.

Photo by Lyn Topinka, 1984 (U.S. Geological Survey).

U.S. Geological Survey volcanologists conduct Electronic Distance Measurement surveys NW of Lassen Peak as part of a monitoring program at Lassen volcano. Chaos Crags lava dome rises in the background. The large scarp at the left was formed during collapse of part of Chaos Crags about 1650 AD.

Photo by Lyn Topinka, 1984 (U.S. Geological Survey).

Lassen Peak, rising above Manzanita Lake on the NW, is the focal point of the Lassen volcanic center, a concentration of volcanic features covering much of Lassen National Park. The massive lava dome forming Lassen Peak was constructed about 25,000 years ago and was the site of California's most recent eruption during 1914-1917. Chaos Crags, a lava dome complex on the north flank, and the aptly named Cinder Cone to the NE, have also erupted within the past 1200 years.

Photo by Dave Wieprecht, 1995 (U.S. Geological Survey).

Lassen Peak rises in the background behind the Chaos Crags, a group of dacitic lava domes on Lassen's north flank. The Chaos Crags were formed during a series of eruptions about 1100 to 1000 years ago in which early explosive eruptions and pyroclastic flows were followed by growth of a complex of five lava domes. The area of light-colored talus on the right side is the source of a large debris avalanche from Chaos Crags about 1650 AD.

Photo by Dan Dzurisin, 1982 (U.S. Geological Survey).

Chaos Crags in the foreground and Lassen Peak in the background are large lava dome complexes in the southern Cascade Range. Chaos Crags consists of a group of five overlapping dacitic lava domes that were erupted a little over 1000 years ago. Lassen Peak was formed about 25,000 years ago, but last erupted during 1914-17.

Photo by Dan Dzurisin, 1982 (U.S. Geological Survey).

The Chaos Crags lava dome complex on the north flank of Lassen Peak, seen here from the south, was formed at the end of an eruptive period about 1100-1000 years ago. A tephra ring from associated explosive eruptions forms the light-colroed area at the middle right.

Photo by Dan Dzurisin, 1981 (U.S. Geological Survey).

The Devastated Area swept by pyroclastic surges during an explosion on May 22, 1915 is seen here from the NE flank of Lassen Peak. The May 22 produced an eruption plume as high as 9 km, a pyroclastic surge that swept the area seen here, and a series of three lahars, the two largest of which swept down Lost Creek to the NE. The area as far as the distant flank of forested Raker Peak at the upper left was affected by the May 22 pyroclastic surge. Revegetation has begun to cover the distal parts of the May 1915 deposits.

Photo by Bill Chadwick, 1981 (U.S. Geological Survey).

Lassen Peak, seen from Brokeoff volcano to the SW, is one of a series of dacitic lava domes erupted during the past 25,000 years along the northern edge of a caldera on the northern flank of Brokeoff volcano. Lassen Peak is the largest and most recently active of these domes.

Photo by Bill Chadwick, 1981 (U.S. Geological Survey).

The summit of Lassen Peak contains a blocky lava flow in the center and several craters formed during the 1914-1917 eruption. Lava flows spilled through low notches in the east and west crater rims. The crater in the right foreground, on the NW side of the summit, was formed as a result of explosions near the end of the eruption in 1917.

Photo by Bill Chadwick, 1981 (U.S. Geological Survey).

In May, 1915, a year after the start of the eruption, two blocky lava flows spilled through gaps in the eastern and western crater rims and descended the upper flanks of the volcano. The western flow, seen here, traveled about 500 m down the flank; on May 19, 1915, the eastern flow fragmented and mixed with snowmelt, forming a mudflow that traveled 35 km down Lost Creek and Hat Creek valleys, destroying bridges and farm buildings.

Photo by Bill Chadwick, 1981 (U.S. Geological Survey).

Snow-capped Lassen Peak is seen here from the summit of Prospect Peak shield volcano at the NE end of Lassen Volcanic National Park. The unvegetated, largely snow-free peaks on the right horizon are the Chaos Crags, a complex of dacitic lava domes last active about 1100 years ago. The Twin Lakes sequence of andesitic lava shields and cones forms the lake-studded area of the Central Plateau in the center of the photo.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

The aptly named Cinder Cone, a symmetrical pyroclastic cone at the NE end of the Lassen volcanic center, is seen here from the NE across Butte Lake. The unvegetated lava flow at the left originated from the cone. Although there is a report of an eruption from Cinder Cone in 1850 AD, recent work suggests that the cone and associated lava flows all formed during a brief eruptive interval lasting at most a few decades about 230-425 radiocarbon years ago. Lava flows traveled to the NE and SE, forming Snag Lake and Butte Lake.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

Butte Lake in NE Lassen Volcanic National Park was formed when the blocky lava flow seen across the lake dammed local drainages. The flow was one of five accompanying the eruption that formed Cinder Cone several hundred years ago. This marks the NW-most extent of lava flows from Cinder Cone.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

The unvegetated pyroclastic cone in the foreground, seen from near the summit of Prospect Peak, is Cinder Cone in NE Lassen Volcanic National Park. Cinder Cone, which was formed during an eruption several hundred years ago, was the source of an extensive series of lava flows that can be seen on the far side of the cone. The flows dammed up local drainages, forming two lakes, one of which is Snag Lake, seen here SSE of the cone.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

Lava flows traveled about 3.5 km to the north and south from Cinder Cone, blocking drainages and forming two lakes. The northern lake, Butte Lake, is seen here from the summit of Cinder Cone. The quartz-bearing basaltic lava flows originated from vents at the SE flank of the pyroclastic cone. The old Emigrant Trail connecting Nevada with the Sacremento Valley winds through the trees at the left base of the cone.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

A series of lava flows from a vent on the SE flank of Cinder Cone traveled about 3.5 km to the north and south. Snag Lake, seen here to the south from the summit of Cinder Cone, was formed with the lava flow dammed up drainages. The colorful area at the lower left, known as the Painted Dunes, is an ash deposit oxidized by the heat of a still-hot underlying lava flow. The dark-colored main flow beyond the Painted Dunes was emplaced later during the same eruption, and is ash free.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

The summit of Cinder Cone contains nested craters with several crater rims created as a result of changes in vent location and eruption intensity. The scoria cone was formed during eruptions several hundred years ago in NE Lassen Volcanic National Park. Prospect Peak, an andesitic shield volcano capped by a small pyroclastic cone, is the forested peak in the background NW of Cinder Cone.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

The colorful Painted Dunes at Cinder Cone in NE Lassen Volcanic National Park were formed when ash deposits from Cinder Cone were oxidized by a still-hot underlying lava flow. The mounded surface of the ash reflects the irregular topography of the underlying Painted Dunes lava flow. A black ash-free lava flow, also erupted from Cinder Cone at a later date during the same eruption, can be seen at the top of the photo.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

A roughly 3-m-high rounded glacial erratic at the Bumpass Hell parking lot is witness to the extensive glacial erosion that has affected much of Lassen Volcanic National Park. Brokeoff volcano in the background is the glacially eroded remnant of a large stratovolcano that formed begining about 600,000 years ago. At its peak the volcano may have reached a height of 3350 m. Glacial erosion of hydrothermally altered rocks at the core of the volcano has produced a large central depression.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

This view from Prospect Peak shows perhaps the youngest lava flow immediately north of the Lassen volcanic center. This sparsely vegetated flow originated from a small cinder cone (left-center) between Prospect and West Prospect (upper left) peaks, two young basaltic lava cones straddling the NE border of Lassen Volcanic National Park. The young, but undated andesitic flow traveled initially to the NE and then around the flank of West Prospect Peak to the NW.

Photo by Lee Siebert, 1998 (Smithsonian Institution).

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

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